1. Field of the Invention
The present invention relates to a focusing state determining apparatus, and more specially, to a focusing state determining apparatus applied for a television camera and so on in which subject light separated in a color separation optical system is imaged by a picture-imaging element for each color.
2. Description of the Related Art
In a device for determining a focusing state of a television camera and a video camera for example, a method is known that a high frequency component is extracted from an image signal captured by an imaging element, and a focus evaluation value representing sharpness (contrast) of the image is obtained from the high frequency component so that the largest (maximum) focus evaluation value is detected.
A wobbling method is also known where a focus lens of a taking lens or the imaging element is vibrated periodically (wobbled) so that the focusing state is determined from the fluctuation of the focus evaluation value.
Concerning the wobbling method, in order to solve the problem in that image quality is deteriorated because of the wobbling of the imaging element and a camera cannot function quickly, Japanese Patent Application Publication No. 8-50227 discloses the following technique. Subject light entering the camera body through the taking lens is generally separated by a color separation optical system and applied to each picture-imaging element for each color. In the color separation optical system, a branch optical path is formed, which is different from optical paths of the subject light applied to the picture-imaging elements. The subject light on the branch optical path is applied to a focus-imaging element different from the picture-imaging elements. Then, the focus-imaging element is wobbled along the optical axis. According to this technique, the image quality is not deteriorated by the wobbling, and a focusing can be quickly performed because the amount of the wobbling can be increased.
In the wobbling method, a plurality of focus evaluation values that are obtained in time series while wobbling are compared to each other so as to determine the focusing state. Hence, the focusing state cannot be determined quickly in the wobbling method disclosed in Japanese Patent Application Publication No. 8-50227. Moreover, there are problems that a construction of the apparatus is complicated, the apparatus is large and the cost is high, since a special driving device for wobbling the imaging element is required.